In my U.S. Pat. No. 4,979,417 issued Dec. 25, 1990 entitled Rotating Saw Blade Having Improved Critical Vibrational Speed, a rotating saw is set forth having a variable thickness circumferential sections. These sections include a relatively thick integral rotating hub which contributes to the support of the saw. This rotating hub does not enter into the sawed kerf made by the saw in cut lumber.
Passing into the produced kerf of my Bird '417 saw construction are two variable thickness circumferential saw portions. The first and inner portion is a relatively thick and relatively narrow circumferential saw portion joined to the hub at its smallest radius and protruding outward toward the saw teeth. The second is a relatively thinner and larger circumferential saw portion joined to the thick and small circumferential saw portion at its inner radius and supporting the saw teeth at its outer radius. The supported teeth are wider than both circumferential sections--but not necessarily wider than the central hub.
The design of this saw is principally tailored to counteract transverse or lateral forces and hoop stresses which act to produce kerf degrading vibrational modes occurring when a rotating saw cuts lumber and produces a kerf. Specifically, in the attempt to produce ever smaller saw kerfs (and waste ever smaller quantities of processed lumber in the form of sawdust) saws have been constructed of thinner dimensions. The design of my '417 Patent enables optimum metal distribution through out a rotating saw to enable optimum resistance to vibrational modes which degrade and rendered wider and less even the intended saw kerf.
As distinguished from my earlier '417 Patent, the present invention relates to linear saws. Such linear saws present considerations that are quite different from those encountered in circular saws. First, such saws are supported at both ends. Typically, the entire length of the saw is under tension. The greater the tension, the greater the ability of the saw to resist transverse forces from the side of the saw (normal to the plane of the kerf).
Secondly, the entire length of the saw between the supports at opposite ends of the saw passes through the kerf of the cut lumber. There is no central hub. Further, vibration in a circumferential (circular) mode is obviously not a consideration. The waves do not propagate in a circular path about the center of the saw. No portion of the saw can be thicker than the saw kerf, as can be the case for the central hub of a circular saw. There is no part of a linear saw analogous to the central hub of a circular saw.
Thirdly, continuous band saws and reciprocating saws, in common with circular saws, do not retain within the tooth gullets all of the material which has been cut. Some of this material (e.g., "sawdust" in wood cutting or "chips" in metal cutting) escapes from the gullets before the saw tooth gullets are able to exit the workpiece and discharge the contents of the gullets.
This effect is more pronounced in linear saws because band and reciprocating saws are most often selected for greater widths of cut. Because of the greater distance which each tooth must travel while cutting, a greater volume of sawdust or chips is generated by each tooth, and a greater volume of that material escapes the gullet into the zone between the sides of the tooth and the surface of the cut. Because of these required sawdust ejection considerations, the thickness of bands supporting the teeth is linear saws is often only slightly more than half of the width of the kerf itself.
Finally, and especially with respect to band saws, so-called gullet cracking--cracking of the saw at the leading edge in the arcuate portion immediate below or above a supported tooth--accompanies fatigue failure of the saw. Such fatigue failure is accentuated in band saws by stress reversal as the saw passes under tension around wheels reversing the path of the saw. This is especially pronounced in newer "high strain" band saws where the saw is under the maximum tension to maintain a linear kerf against the forces of transverse deflection.
It should be noted that in linear saws, transverse deflection of one kind or another is to be anticipated. Such transverse deflection most frequently occurs when pieces of already severed material find their way into the interval between the kerf and saw or when material of varying density is encountered. Other forces, such as unwanted deflections and vibrations can subject linear saws to transverse forces. It is to be understood that the resistance to such transverse forces is one of the main limiting factors in linear saw design.
In the following description, reference to the particular portions of linear saws will be required. Viewing such a saw from the side with the teeth disposed in a vertical line at the left will be presumed. Reference will be made to the saw backing. This is the metal behind the teeth that support the teeth in cutting. It will be appreciated that this metal backing must of necessity pass through the kerf created by the saw.
The term "thickness" will be utilized. This will refer to the dimension of the saw through the teeth or backing in a direction normal to the major plane of the saw. Further, the term "longitudinal" is utilized. This refers to vertical segments of the saw backing taken parallel to the line of the saw teeth. Additionally, the term "width" is used. This refers to the dimension D (see FIG. 1) of the saw in the cutting direction. This width dimension is taken from the cutting teeth to end of the saw backing. The term "depth" is usually reserved for the thickness of the material being cut or the distance along which the saw is cutting (also known as "cutting height") .